ORPHEUS User’s Manual
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3. PUMP REQUIREMENTS
Good performance of ORPHEUS requires high pump quality in terms of both time and space
coherence. This means that ideal pump is diffraction-limited beam and transform-limited, high contrast
pulse. To some extent, performance of ORPHEUS is an indicator of quality of the pump laser radiation.
In femtosecond systems that use nowadays-standard chirped pulse amplification technique (CPA),
there are specific possible causes of distorted beam or pulse. Some of them are discussed below.
Spatial beam quality. Non-uniformity of the beam reduces the energy conversion rate. Presence of
hot spots in the beam may cause small-scale self-focusing that in turn leads to phase modulation. In the
nonlinear crystals, hot spots can also produce parametric superfluorescence or white light generation.
Increasing the average power in such cases could lead to damage of the crystal or other optical
components.
Spatial/temporal beam distortion. In contrast to long pulse lasers, astigmatism introduced by
improperly aligned lenses of beam expanders/reducers lead to distortion of temporal profile of the pulse
across the beam. This makes it impossible to overlap pump and signal pulses over entire beam aperture in
ORPHEUS.
Tilted pulses. This phenomenon manifests itself in similar way as that discussed above. However, it
originates from improper pulse compressor alignment. Tilted pulses are produced when the angular
dispersion is not completely cancelled. The problem with this kind of distortion is that it can be easily
overlooked using standard diagnostics equipment such as autocorrelator.
Underseeded regenerative amplifiers. The seed pulses for regenerative amplifier can be too week
due to low oscillator power, poor mode matching or improper Pockel’s cell timing. This results in rather
high energy content in the background. Background can be undetectable either by oscilloscope or by
autocorrelator. Simple test for background is measurement of build-up time of unseeded and seeded
regenerative amplifier. In last case the build-up time should be shorter at least by 15-20%.
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